The present invention relates to compositions and fluids for use in drilling, fracturing and completing operations and, more particularly, to oil-in-water emulsion well serving fluids.
In the drilling or the treatment of earth boreholes such as oil and gas wells, fluids or muds find an important use. For example, in the rotary drilling of wells, a drilling fluid is continuously circulated from the surface of the ground to the bottom of the well borehole and back to the surface of the ground again. The drilling fluid has various functions including those of lubricating the drill bit and pipe, carrying cuttings from the bottom of the well borehole to the surface of the ground, and imposing a hydrostatic head in the drilled formation to prevent escape of oil, gas, or water therefrom into the well borehole during the drilling operation. Ordinarily, aqueous drilling fluids comprising a suspension of a clay in water are employed. However, in certain cases such aqueous drilling fluids can be particularly disadvantageous.
It is not uncommon, depending on the area in which the drilling is being conducted, to encounter subterranean formations made up primarily of hydratable shales, silts or clays. When such hydratable materials are contacted with an aqueous based drilling fluid, they tend to rapidly disperse in the aqueous drilling fluid. The dispersion of the shale or clay into the drilling fluid results in damage to the borehole wall, increases the solids content of the returning drilling fluid, and makes it difficult to remove the dispersed solids from the drilling fluid. Also if shales and clays are wetted by the aqueous drilling fluid they will tend to form a sludge in the fluid, raising its viscosity.
It is common, in an attempt to overcome the disadvantages of water based drilling fluids, to employ drilling fluids having a fluid phase consisting entirely of oil or an emulsion of oil-in-water. Of these two types, the emulsion drilling fluids are usually preferred. Oil-in-water emulsion drilling fluids have been used extensively. In such drilling fluids, water, the predominant liquid component, is the continuous phase of the emulsion while the oil, dispersed in the water phase, is the internal phase of the emulsion. It is known to use certain oil-in-water emulsions containing, for example a blended anionic-non-ionic emulsifier, for drilling through hydratable shale or clay formations. Such emulsions, to some extent, inhibit surface hydration of the formations preventing damage to the wall of the well bore and dispersion of the cuttings in the returning drilling fluids.